In automotive and other industrial applications, special sensors are used to determine shaft speed and angular position, for example, as well as linear motion sensing. Generally such sensors are either of the variable reluctance variety or comprise a toothed wheel (i.e., exciter) spaced from a sensor comprising a magnet and a magnetoresistor or a Hall-effect device. Other types of sensors require multi-bit digital encoding for position sensing and other uses.
A permanent magnet with an appropriate magnetization pattern can serve as the exciter component of a magnetoresistive sensor without the need for a separate bias magnet. However, by conventional production methods currently in use, very small magnet exciters could not be magnetized with a pattern providing the necessary combination of resolution and field strength, and the cost of a large permanent magnet exciter would be prohibitive. If several different magnetization patterns are desired side by side, such as for multi-bit digital encoding, more complex manufacturing problems arise; either machining or magnetizing such an exciter as one unit is very costly and is seldom done.
It has been proposed in U.S. Pat. No. 4,312,684 to Chraplyvy et al entitled "Selective Magnetization of Manganese-Aluminum Alloys" and in U.S. Pat. No. 4,347,086 to Chraplyvy et al entitled "Selective Magnetization of Rare-Earth Transitional Metal Alloys", both assigned to the assignee of the present invention, to create local regions of hard magnetic material in a body or substrate of a special nonmagnetic or soft magnetic material by exposing selected portions or regions of the substrate to a laser beam for heating such portions or regions to a transformation temperature at which magnetic material is formed. The magnetic regions are magnetized in a strong field to produce a permanent magnetic code having sufficient flux density to be readable with a magnetic sensor such as a magnetic tape head. The materials used are expensive and the magnetic fields produced are very weak.
In addition, the paper of Ara et al, "Formation of Magnetic Grating on Steel Plates by Electron/Laser Beam Irradiation", IEEE Trans. Magnetics, Vol. 25, No. 5 (1989), p. 3830, discloses an attempt to make a magnetic sensor by forming magnetic gratings on nonmagnetic austenitic stainless steel by laser beam heating of strips on the plate to a temperature sufficient to effect transformation of the heated regions to produce small grains of the ferromagnetic phase in the austenitic phase, and similarly heating a ferromagnetic carbon steel having a ferrite/pearlite phase which was changed to martensite by beam irradiation. The gratings were magnetized and the magnetic flux from each track was detected by a sensor passed over the grating. The signal produced was far too weak to be useful in many applications.
It has also been proposed to alter the magnetic properties of very thin films of special materials for data storage by a thermomagnetic method. In the recording of a magneto-optical disc, the thin layer (about 1 .mu.m thick) of an amorphous transition metal-rare earth alloy is coated on a disc and the entire disc is magnetized in a given direction. A laser is then used to locally heat the surface (typically a 1.6 .mu.m diameter spot) in a static-applied magnetic field to reverse the direction of the disc's magnetization in the heated regions. Because the magnetic regions are so small and magnetically weak, a magnetic sensor such as a magnetoresistor or a Hall-effect device cannot respond to the individual bits of data except from extremely small distances or air gaps which are highly impractical. The data is read optically using the Kerr effect. This requires a beam splitter, two detectors, two linear polarizers, a half-wave plate and beam steering optics. The delicate and complex nature of the detection optics precludes this type of magneto-optical recording from forming the basis of a viable automotive sensor.
Methods which utilize laser beam heating require the use of collimators to narrow the energy beam to a limited diameter or cross-section. Such methods are limited to heating one selected region at a time with the carefully collimated beam so as to maximize heating of selected regions of a body while minimizing heating of any unselected regions. Thus, the cross-section of the energy beam must be carefully controlled and the beam itself must be carefully directed onto each selected region of the body, one region at a time. Despite such careful control, undesired lateral flow of heat in the body from selected to unselected regions occurs. Further, such methods are relatively time consuming and not an optimal method for mass production.
Thus, such methods are restricted by the limited ability to direct the pattern of heat and by the inability to minimize the lateral flow of heat. Accordingly, these limitations restrict the ability to densify the pattern, or closely space the selected and unselected regions.
Therefore, it is desirable to have an improved method for heating and imposing a field which minimizes heating of unselected regions without diminishing the strength of magnetic characteristics carried by the heated selected region. It is also desirable to have such an improved method suitable for use with magnetic bodies of any thickness, including thin film, disc and bulk magnetic bodies, to thereby enable dense patterns to be produced in any magnetic body conveniently, efficiently and economically.